The formation mechanism of linear-carbon-chain molecules, C n O ( n = 2 - 9), synthesized in the discharge of C 3O 2 has been investigated on the basis of detailed analyses of previously obtained FTMW spectroscopic data. The relative abundances of the C n O products determined from their rotational spectrum intensities agree with those for the C n O (+) ions. The active chemicals in the reaction system include :C and :CCO only, and the observed products exclusively consist of C n O, leading to a likely formation mechanism of the atomic-carbon addition and ring opening reaction. This formation mechanism is simple and efficient, and it is applicable not only to linear-carbon-chains but also to a wide range of carbon processes, in particular, to ultra low temperature or incomplete combustion conditions.